1. Field of the Invention
The present invention relates to calibration of in-phase and quadrature branches of a quadrature transmitter or transceiver, such transmitter or transceivers being based on a zero-IF or low-IF architecture. Such transmitters or transceivers can devices under the s-called IEEE 802.11a or IEEE 802.11b Standard, can be cell phones, or can be any other suitable radio frequency device.
2. Description of the Related Art
Transmitters or transceivers based on a zero-IF (Intermediate Frequency) or low-IF architecture are well-known in the art. Such transmitters have an in-phase and quadrature transmit branch and an up-converter to up-convert a base band signal to a radio frequency signal, typically by a pair of quadrature mixers. Such zero-IF or low-IF transmitters are usually implemented as an integrated circuit on a chip. Due to asymmetrical layout of diverse components of the transmitter on the chip, such as off-center positioning of transmit filters or other components, mismatch occurs. Such a mismatch produces DC-offsets in the transmit branches, before up-conversion, an herewith undesired local oscillator (LO) components in up-converted signals. Such unwanted LO components, also known as LO-feedthrough, might be of such a severe nature, depending on component and IC-process spread, that customer requirements can no longer be fulfilled.
It is an object of the invention to provide an effective and optimal method of calibrating a quadrature transmitter.
In accordance with the invention, a method of calibrating a quadrature transmitter is provided, said method comprising:
injecting a first calibration signal into an in-phase transmit branch of said quadrature transmitter;
injecting a second calibration signal into a quadrature transmit branch of said quadrature transmitter, said first and second calibration signals being injected before performing up-conversion in said transmitter and being produced by respective first and second digital signals;
detecting an up-converted signal;
digitizing said detected up-converted signal; and
calibrating said in-phase and quadrature transmit branches by alternately determining said first and second calibration signals while at least varying respective most significant bits of said first and second digital signals, upon said varying said at least most significant bits keeping calibration bit values that correspond to minimum values of said digitized detected up-converted signal.
The invention is based on the insight to first and quickly remove large errors, such as a major part of DC-offset errors, in in-phase and quadrature transmit branches, thereby avoiding otherwise unnecessary repetitious calibration cycles.
Preferably and very advantageously, an alternating transmit branch calibration scheme, as applied to the most significant bits of the digital signals that produce the calibration signals, is also and successively applied to less significant bits of the digital signals.
In order to check whether a still better calibration result may be obtained, neighboring values of a determined calibration signal are tried, and the best result of such a check is taken as the final calibration signal.
In case of a low-IF transmitter architecture, a single tone calibration signal is input to the transmitter, and a filter is arranged before the detector so as to filter out an up-converted tone signal and to suppress a local oscillator signal.